The present invention relates to a method and apparatus for projection exposure used for forming fine patterns in semiconductor integrated circuits, liquid crystal displays, etc., by photolithography processes. More particularly, the present invention relates to a method and apparatus for projection exposure which may be effectively applied to a process for forming contact hole patterns which are close to each other.
A projection exposure apparatus is used for forming fine patterns in semiconductor integrated circuits, liquid crystal displays, etc., by photolithography processes. A projection optical system used in such a projection exposure apparatus is incorporated in the apparatus after high-level optical designing, careful selection of a glassy material, ultraprecise processing, and precise assembly adjustment. The present semiconductor manufacturing process mainly uses a stepper in which a reticle (or a photomask or the like) is irradiated with the i-line (wavelength: 365 nm) of a mercury-vapor lamp as illuminating light, and light passing through a circuit pattern on the reticle is focused on a wafer (or a photosensitive substrate), which has a photoresist coated thereon, through a projection optical system. Recently, an excimer stepper that employs excimer laser light (KrF laser light of wavelength 248 nm) as illuminating light has also been used.
Generally speaking, to faithfully transfer a fine reticle pattern onto a photosensitive substrate by exposure using a projection optical system, the resolution and focal depth or depth of focus (DOF) of the projection optical system are important factors. Among projection optical systems which are presently put to practical use, there is a projection optical system having a numerical aperture (NA) of about 0.6 and employing the i-line. In general, when the wavelength of illuminating light employed is kept constant, as the numerical aperture of the projection optical system is increased, the resolution improves correspondingly. In general, however, the focal depth (DOF) decreases as the numerical aperture NA increases. The focal depth is defined by DOF=.+-..lambda./NA.sup.2, where .lambda. is the wavelength of illuminating light. It should be noted that when the illuminating light is reduced in wavelength, the resolution improves, but the focal depth decreases with the reduction in wavelength in the same way as in the case of the numerical aperture.
In the meantime, even if the resolution is improved by increasing the numerical aperture NA of the projection optical system, the focal depth (focus margin) DOF decreases in inverse proportion to the square of the numerical aperture, as shown in the above expression of DOF=.+-..lambda./NA.sup.2. Accordingly, even if a projection optical system having a high numerical aperture can be produced, the required focal depth cannot be obtained; this is a great problem in practical use. Assuming that the wavelength of illuminating light is 365 nm of the i-line and the numerical aperture is 0.6, the focal depth DOF becomes a relatively small value, i.e., about 1 .mu.m (.+-.0.5 .mu.m) in width. Accordingly, resolution decreases in a portion where the surface unevenness or the curvature is greater than DOF within one shot region (which is about 20 by 20 mm or 30 by 30 mm square) on the wafer.
To solve the above-described problems of the projection optical system, there have been proposed a phase shift reticle method (which uses a reticle provided with a phase shifter) such as that disclosed in Japanese Patent Examined Publication No. 62-50811, and a SHRINC (Super High-Resolution by Illumination Control) method such as that disclosed in Japanese Patent Unexamined Publication (KOKAI) Nos. 04-101148 and 04-225358. These methods are suitable for improving the resolution and the depth of focus in the process for transferring a pattern formed on the reticle, particular a periodic pattern, e.g., a line-and-space pattern (L&S pattern) or a grating pattern. However, no effect can be obtained for isolated patterns (in which the distance between patterns is relatively large), for example, contact hole patterns (fine square patterns).
Therefore, in order to enlarge the apparent focal depth for isolated patterns, e.g., contact hole patterns, an exposure method has been proposed in which for each shot region on a wafer, the wafer is stepwisely moved along the optical axis by a predetermined amount at a time, and exposure is carried out for each stop position of the wafer, that is, exposure is carried out a plurality of times for each shot region. For example, see Japanese Patent Unexamined Publication (KOKAI) No. 63-42122 (corresponding to U.S. Pat. No. 4,869,999). This exposure method is called FLEX (Focus Latitude enhancement EXposure) method and provides satisfactory focal depth enlarging effect for isolated patterns, e.g., contact hole patterns. However, the FLEX method indispensably requires multiple exposure of contact hole pattern images which are slightly defocused. Therefore, both the sharpness of a composite optical image obtained by the multiple exposure and a resist image obtained after development inevitably decrease. Accordingly, the FLEX method suffers from problems such as degradation of the resolution of contact hole patterns which are close to each other, and lowering of the margin for the variation of the exposure degree (i.e., exposure margin).
It should be noted that the FLEX method is also disclosed in Japanese Patent Unexamined Publication (KOKAI) No. 05-18805 (corresponding to U.S. patent application Ser. No. 820,244, filed by the present applicant; in which the wafer is moved along the optical axis continuously during exposure), and U.S. Pat. No. 5,255,050.
As another conventional technique, there has recently been proposed a technique in which a pupil filter is provided in a pupil plane of a projection optical system, that is, a plane of the projection optical system that is in Fourier transform relation to both the reticle pattern surface and the wafer surface, in an image-forming optical path between the reticle and the wafer, thereby improving the resolution and the focal depth. Examples of this technique include the Super-FLEX method published in Extended Abstracts (Spring Meeting, 1991) 29a-ZC-8, 9; The Japan Society of Applied Physics. This method is also disclosed in EP-485062A. In the Super-FLEX method, a transparent phase plate is provided at the pupil of a projection optical system so that the complex amplitude transmittance that is given to image-forming light by the phase plate successively changes from the optical axis toward the periphery in the direction perpendicular to the optical axis. By doing so, the image that is formed by the projection optical system maintains its sharpness with a predetermined width (wider than that in the conventional method) in the optical axis direction about the best focus plane (a plane that is conjugate with respect to the reticle) which is the center of said predetermined width. Thus, the focal depth increases. It should be noted that the pupil filter used in the Super-FLEX method, that is, so-called multifocus filter, is detailed in the paper entitled "Research on Imaging Performance of Optical System and Method of Improving the Same", pp. 41-55, in Machine Testing Institute Report No. 40, issued on Jan. 23, 1961. As to the pupil filter itself, please see U.S. Pat. No. 5,144,362.
The present applicant has proposed in Japanese Patent Unexamined Publication (KOKAI) No. 04-179958 a pupil filter for contact hole patterns which is designed to block the light passing at the central portion (in the vicinity of the optical axis) of the pupil plane, unlike the above-described filter that gives a change in phase of the transmitted light.
However, the conventional Super-FLEX method suffers from the problem that the intensity of a subsidiary peak (ringing) which occurs in the vicinity of a contact hole pattern becomes relatively strong, although the method provides satisfactory focal depth increasing effect for isolated contact hole patterns. Therefore, in the case of a plurality of contact hole patterns which are relatively close to each other, an undesirable ghost pattern is transferred to a position where ringings occurring between adjacent contact holes overlap each other, causing an undesired reduction in film thickness of the photoresist.
In this regard, the present applicant made analysis in order to determine optimal conditions for projection exposure when carried out by jointly using the above-described various pupil filter techniques and the FLEX method, and disclosed the results of the analysis in Japanese Patent Application Nos. 05-175164 and 05-175165 (corresponding to U.S. patent application Ser. No. 274,752, filed by the present applicant). These inventions provide a method which is extremely useful for the transfer or the projection exposure of contact hole patterns which are relatively close to each other, as described above. In this method, however, the exposure process comprises a plurality of divided exposure operations which are successively carried out for each shot region, and a plurality of exposure positions for the exposure process are uniquely determined by the wavelength of exposure light and the NA (Numerical Aperture) of the projection optical system. Therefore, a depth of focus which is larger than the optimal depth of focus may be obtained, or the amount of exposure (exposure time) required for the pattern transfer may increase according to the conditions. Consequently, the processing capacity (throughput) reduces disadvantageously.